The central aim of this study was to examine the effectiveness of Group Stepping Stones Triple P (GSSTP) in an Irish context for families of children with both developmental disabilities and internalising and externalising behavioural problems. Parents of 84 children (mean age = 5.73; SD = 2.06) with developmental disabilities and co-occurring behaviour problems attending Irish public health services were randomly assigned to a 9-week GSSTP group or a waiting list control (WLC) group. All parents completed self-report measures before (Time 1) and after (Time 2) the programme and parents in the GSSTP group were assessed at 3- to 5-month follow-up (Time 3). At Time 2, clinical improvement and reliable change rates on the primary dependent variables (summary scales of the Developmental Behaviour Checklist and Strengths and Difficulties Questionnaire) were significantly higher in the GSSTP group than in the WLC group. At Time 2, mean scores of the GSSTP group showed significant, small to medium improvements relative to the WLC group on parent-reported child behaviour problems, parenting skills and confidence, and parental adjustment. Most of these improvements were maintained at 3- to 5-month follow-up. These results indicate that GSSTP is a promising intervention for improving child behaviour and parenting outcomes in a mixed-disability group in an Irish context.